10,679 patients who were admitted to hospitals in October spent time on a trolley before securing a bed, the Irish Nurses and Midwives Organisation has said.
The INMO said this was a more than 25% increase on last October, and double the figures for 2020.
Over 393 children spent time on trolleys in October - the worst month for paediatric overcrowding on record according to the INMO
New trolley figures released to RTÉ's This Week show that 1,.342 patients were on trolleys at Cork University Hospital in October, with 1,268 patients spending time on trolleys in University Hospital Limerick and 810 patients in University Hospital Galway.
702 patients spent time on trolleys at St James's Hospital in Dublin and 700 patients did so at Sligo University Hospital.
Phil Ní Sheaghdha, General Secretary of the INMO, told This Week that the number of children waiting on trolleys was "very concerning".
"We didn't used to count children because it wasn't a phenomenon up to 2018. Since 2018 we have been counting and the numbers are getting higher," she said.
Ms Ní Sheaghdha said the new national children's hospital in Dublin would not be a cure-all for overcrowding in the current hospitals.
"The problem is there aren't going be a huge amount of additional beds. The layout is different. We've a lot more single rooms and that requires a lot more staff."
She said not enough paediatric nurses were being trained or retained.
"We keep making the same mistakes over and over again. We do a really good job when we train our medical staff. They work very hard in our Irish public hospitals and then they are poached," she said.
"We should be treating them better, both in training and when they qualify."
She said she fundamentally agreed with the Health Information and Quality Authority that keeping sick people on trolleys was "inhumane" and a safety concern.
"I've been in a number of emergency departments in the last three weeks. At one point I couldn't walk between two trolleys. I was thinking how are we going to get emergency equipment through here," she said,
The INMO has asked the Health and Safety Authority to look again at the safety of staff and patients in emergency departments.
The INMO General Secretary said care needed to prioritised to combat overcrowding. "We can only cope with one service, emergency service or elective service, we can't do both."
She said people waiting for elective procedures should be looked after in private hospitals.
Ms Ní Sheaghdha also repeated a call mandatory mask wearing in order to prevent the spread of Covid-19 in hospitals.
She said that hospital-acquired illness was on the rise.
Sinn Féin's health spokesperson David Cullinane has described the figures as "deeply disturbing".